Figure 1: UV/ozone
mass cleaning

Cleaning of platinum-iridium mass standards using ultra violet light and ozone provides a non-contact alternative to solvent/steam cleaning.

NPL scientists have developed an apparatus for cleaning mass standards that uses a combination of ozone and ultra violet light (Figure 1). UV light of wavelength 185 nm creates the ozone which bonds with the hydrocarbon contamination on the surface of the mass. A second UV wavelength at 250 nm breaks these carbon-oxygen bonds, releasing the contamination from the surface in the form of CO₂ (P J Cumpson and M P Seah, Metrologia 33 (1996) 507).

Figure 2: Mass
steam cleaning

Cleaning of a platinum-iridium kilogram mass standard is necessary as contaminant build up on its surface can typically be 3 µg per year. The current cleaning method involves rubbing each standard with chamois leather soaked in solvent followed by removal of the solvent with a jet of steam (Figure 2). As the current method is operator dependent, it is difficult to reproduce and can introduce instability in a mass standard.

NPL's UV/ozone technique has been evaluated by exposing Platinum foil that had gained contamination over three years to UV light over a range of intensities and duration. X-ray photoelectron spectroscopy (XPS) was used to measure the amount of contamination removed from the foil's surface. The results give an optimum UV intensity of around 12 mWcm^-2 and exposure time of 1 to 1.5 hours (Figure 3). This exposure removed about 5 µg of contamination from the foil's surface (equivalent to a contamination thickness of 2 nm).

Figure 3: UV dirt removal

Further XPS measurements were made on a platinum foil cleaned in boiling water to compare the effectiveness of the technique with conventional cleaning (Figure 4). Approximately 3.5 µg of contamination was removed with this cleaning process.

Figure 4: Boiling water dirt removal

Following successful trials with platinum foil, the UV/ozone cleaning equipment has been modified to enable cleaning of Platinum-Iridium (Pt-Ir) mass standards (Figure 5). A Pt-Ir mass standard with surface hydrocarbon contamination will be cleaned to confirm the validity of the technique.

Figure 5: Platinum-Iridium (Pt-Ir) mass